Known power supplies convert an input power signal having a given set of parameters (voltage-current-power) to a different, preferably stable set of parameters required by the electronic component or circuit fed by the power supply. Modern power supplies use inverters having a switching transistor or transistors which chop the input signal at a high frequency. The chopped signal is applied to the input or primary side of a step-down or step-up transformer. At the output or secondary side of the transformer, the stepped-down or stepped-up signal is rectified and filtered to obtain the desired output. The transformer isolates the secondary side of the converter from the primary side.
As the load driven by the output changes, or as the input power changes, the output voltage also will change, in the absence of a control circuit to regulate operation of the converter. Known systems use pulse width modulation (PWM) feedback techniques to implement the desired control. In general, the output voltage is sensed and compared to a known reference voltage. Depending on the difference between the two voltages, the duration of a square wave pulse is adjusted to alter the on time of the switching transistors. For example, as load is removed or input voltage increases, a slight rise in output voltage will signal the PWM control circuit to deliver shorter pulses to the switching transistors, thereby decreasing the on time of the transistors and lowering the output voltage. Conversely, as the load is increased or input voltage decreases, longer pulses will be supplied to the switching transistors.
As noted above, the main source of isolation between the primary and secondary sides of the power supply is the power transformer. Nevertheless, there necessarily is interaction between the secondary and primary sides by way of the feedback loop through the PWM control circuit. Isolation also must be supplied in the feedback loop.
If isolation is provided between the feedback tap at the output side and the PWM control circuit, the feedback is referred to as "primary side control" because the PWM control circuit is in direct electrical contact with the primary side of the converter. Supplying start-up power to the PWM circuit is simple because of the availability of a direct path to the input line, while still maintaining isolation between the primary side and the secondary side. The output voltage signal is transmitted from the secondary side to the PWM circuit across the isolation boundary, such as through an optical isolator or a magnetic isolator. As compared to a direct feedback line, the signal applied to the PWM circuit is noisier and slower in response time. In addition, the isolation component can be sensitive to temperature changes and electromagnetic radiation, in addition to being less stable and of limited bandwidth.
To avoid the problems associated with primary side control, isolation can be provided between the PWM control circuit and the power inverter, known as "secondary side control" because the PWM control circuit is in direct electrical contact with the secondary side of the converter. In that case, the problems associated with the isolation component in the feedback line between the output and the PWM control circuit are eliminated, but a separate power supply driven by the primary side and still isolated from the secondary side must be provided to power the PWM circuit. In a representative circuit, a separate power transformer is used for auxiliary power, introducing an additional heavy and bulky element into the circuit.
Sometimes, the application for the circuit will dictate whether primary side or secondary side control is used, despite the disadvantages. For example, it may be necessary to provide an additional control signal to the PWM control circuit, coupled to either the primary side or the secondary side, to enable or disable the power supply. If such a signal is coupled to the primary side, primary side control must be used, whereas if such a signal is coupled to the secondary side, secondary side control must be used.